This invention relates to the field of insulated containers. More specifically, it relates to a class of containers often referred to as vacuum ware which class includes thermos bottles, pitchers, carafes, and the like. A vacuum insulated container, such as a thermos bottle, is one of the most efficient thermal barriers presently known which can be produced at a reasonable cost for wide consumer distribution. Typically, a vacuum insulated container employs a glass or steel envelope or filler from which air is substantially evacuated. The liquid or semi-liquid to be thermally insulated is placed inside the filler to prevent heat loss or gain depending upon the initial conditions of the food stuff as compared with the ambient temperature.
In most vacuum constructions, particularly those which are intended to carry hot soup, coffee, cold beverages, and the like, it is desirable to provide a metallic coating or layer inside the walls of the vacuum filler. The metallic coating, usually silver, acts as a reflective barrier to reduce heat loss by radiation in the infrared region. This approach is commonly used when the filler is formed of glass. Metal fillers are also known. Such fillers are usually formed from stainless steel and loosely packed material is provided in the evacuated interior of the filler to perform a gettering function aiding in the maintenance of the vacuum. Charcoal, carbon black, and metal powders are common ingredients utilized for this purpose.
Other types of insulated containers which are in popular use include foam insulation, such as styrofoam or polyurethane. By comparison with vacuum bottles, however, foam is far inferior in its thermal insulating properties and, for example, will not satisfactorily maintain coffee at serving temperature over the course of a working day as will a vacuum insulated thermos bottle.
With the advent of the microwave oven and its wide consumer acceptance, there arises a need to provide a line of vacuum insulated containers which can be used in conjunction with microwave ovens. This would greatly facilitate the preparation of hot liquids at home for use at work or at school during the course of a day. Thus, soups, coffee or other liquids which may be stored in a refrigerator or at room temperature can be poured in a vacuum insulated container and then placed in a microwave oven for rapid heating. The container itself must be made from a material which will not adsorb microwave energy.
While the foregoing use of thermos bottles is highly desirable, unfortunately none of the designs presently on the market is suitable for use in microwave ovens. The all metal bottles may not be used in microwave ovens because they adsorb significant energy from joule heating due to eddy currents formed in the metal. Also, they may damage the microwave oven magnetron tube by reflecting the energy back down the wave guide. Glass fillers which are provided with a silver reflective coating are similarly unacceptable .
Accordingly, it is an object of the present invention to provide a new class of vacuum insulated containers which are safe for use in microwave ovens.
Another object of the invention is to provide a new class of vacuum insulated containers which have high thermal retention capabilities but which do not adsorb energy at microwave frequencies.
A further object of the invention is to provide a class of vacuum insulated serving pieces in which food can be heated in a microwave oven and then served, which serving pieces do not readily increase in temperature by drawing heat from the food placed therein.
Another object of the invention is to provide a class of vacuum insulated containers composed of materials neither electrically conductive nor made from materials which adsorb energy at microwave frequencies.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.